The invention described herein may be manufactured and used by or for the Government of the United States for all to governmental purposes without the payment of any royalty.
The present invention relates generally to optical elements, and more specifically to optical elements having high damage resistance and tolerance.
A typical optical element requiring high damage resistance and tolerance is an optical limiter. Optical limiters are generally transparent optical elements that block, or limit to a maximum intensity, the transmission of incident light at specific wavelengths. A primary use for optical limiters will be for protection against laser radiation.
Optical limiters may be based on a number of different physical processes, such as scattering, absorption or reflection.
Optical limiters based on absorptive processes can be optimized by controlling the geometrical location of an absorbing species both longitudinally and transversely inside the limiter so that, for example, greater amounts of the absorbing species are located at an expected focal region for incoming laser irradiation. Such optimized optical limiters, however, are subject to optically induced damage inside that focal region. In solid matrices, the damage mechanism may be thermomechanical or thermochemical. Thermomechanical damage occurs by conversion of laser energy to heat at the site of an absorbing inclusion. The rapid heating process at the inclusion causes a thermomechanical fracture to occur in the surrounding matrix or host. The resulting small fracture site is a source for scattering and results in an irreversible damage site.
Prior art optical limiters commonly use solid polymeric plastic host matrices with dopants to produce optimized optical limiters. An example solid polymeric plastic host material is polymethylmethacrylate (PMMA). Unfortunately, the susceptibility of these plastic-based optical elements to damage reduces the range over which this type of solid limiter may be used.
One method for overcoming the problem of permanent damage in a host matrix is to use a liquid limiter based on a chromophore dissolved in a solvent. A chromophore is that portion of a dye molecule that gives it its color and is usually the most fragile part of the molecule. The advantage of liquid limiters is that they are damage tolerant and can undergo xe2x80x9cself-healingxe2x80x9d once damaged. The self-healing process occurs when the damage site, usually in the form of a bubble, floats away and new solution takes its place. A primary disadvantage of liquid limiters is that their performance cannot be optimized by control of the geometrical placement of absorbing species since liquids cannot preserve a concentration distribution. Liquids are also subject to leaking and other types of failures.
Other rigid host matrix materials exist which exhibit increased damage resistance compared to other polymer materials.
Unfortunately, despite the high damage resistance of some rigid host matrix materials, their operational range as provided through damage resistance is still limited. Further, they exhibit little damage tolerance. Damage tolerance is not the same as damage resistance. Damage resistance indicates the ability to accept higher radiation levels without any apparent effect. Damage resistance can only be increased to some level and then irreversible permanent damage will occur. Damage tolerance, however, as exhibited by, for example, liquid optical limiters, indicates the desirable ability to allow damage to occur temporarily and then subsequently heal. While there is a limit, of course, to damage tolerance before irreversible damage will occur, adding a damage tolerance capability to damage resistance should substantially increase the operational range of optical limiters.
Thus it is seen that there is a need for optimized optical limiters having increased damage resistance and a further need for optimized optical limiters having increased damage tolerance.
Another optical element requiring high damage resistance and tolerance are the gain or lasing media in solid state dye lasers, particularly dye-doped polymer host materials. Such solid state dye lasers are an attractive alternative to more common liquid dye lasers (which use complex organic dyes such as rhodamine 6G in liquid solution or suspension as lasing media), providing the tuning and other advantages of liquid dye lasers without such problems as sealing and the size and complexities of pumping. Polymer host matrices are particularly attractive because, among other reasons, they can be easily doped with dyes at high concentrations. Unfortunately, solid state dye lasers using polymer host materials are severely limited in power output by low damage thresholds.
U.S. Pat. No. 5,610,932 to Kessler et al., which is incorporated by reference into this description, discloses a solid state dye laser host made of a gel material. The use of a gel material appears to result in xe2x80x9cxe2x80x98self-healingxe2x80x99 after photobleaching due to dye migration,xe2x80x9d thus avoiding many of the disadvantages of liquid dye lasers without forfeiting as much power output as in most solid state dye lasers. The Kessler et al. invention nevertheless will not be able to achieve the same power outputs as liquid dye lasers.
Thus it is seen that there is also a need for solid state dye lasers having increased damage resistance and tolerance and thus capable of higher power outputs.
Many other, if not most, optical elements will benefit from new apparatus and methods for increasing damage resistance and tolerance to the transmission of light energy through those optical elements.
It is, therefore, a principal object of the present invention to provide optical elements having high damage resistance and tolerance.
It is a feature of the present invention that it provides a nonuniform distribution of dopants, such as light limiting dopants for an optical limiter or lasing media for a solid state dye laser, inside a host material.
It is another feature of the present invention that it adds gradient mechanical properties to optical elements.
It is an advantage of the present invention that it has an ability to form and preserve a dopant concentration distribution not possible in liquids.
It is another advantage of the present invention that it resists agglomeration of particulate dopants, thus providing longer shelf life and more durability against environmental extremes than liquid, solvent-based optical elements.
It is a further advantage of the present invention that it allows both the placement of appropriate dopants and the gradient mechanical properties of a host matrix material to be optimized for maximum performance.
It is a still further advantage of the present invention that it is safer than liquid optical elements where a glass cell may fracture with a resultant spill of hazardous solvents.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.
In accordance with the foregoing principles and objects of the present invention, various embodiments of a novel optical element are described. The unique discovery of the present invention is that combining gradient mechanical properties with an optimizable geometrical placement of light altering dopants inside a host matrix results in an efficient, optimizable and highly damage resistant and tolerant optical element. For an optical limiter embodiment, the light altering dopants are absorbing species or other light limiting dopants. For a solid state dye laser embodiment, the light altering dopant is a lasing or gain media. The viscoelastic properties of the host matrix material can be controlled by controlling crosslink density of a crosslinked polymer which, in the optical limiter embodiment, combined with geometrical placement of absorbing species (generally a greater concentration of absorbing species in regions of softer matrix material), makes an optical limiter having a very high laser damage threshold.
Accordingly, the present invention is directed to an optical element, comprising an elastic host material, a light altering dopant inside the elastic host material, and a nonuniform concentration distribution of the light altering dopant inside the elastic host material. The concentration of light altering dopant may increase toward the center of the host material. The concentration of light altering dopant may also increases toward a preselected focal region inside the elastic host material. The optical element may further include regions of different stiffness within the elastic host material, and may also include a gradient stiffness distribution within the elastic host material. The stiffness may decrease toward a preselected focal region inside the elastic host material. The concentration of the light altering dopant may increase toward regions of lesser stiffness.
The present invention is also directed to an optical element, comprising a elastic host material, a light altering dopant inside the elastic host material, a nonuniform concentration distribution of the light altering dopant inside the elastic host material, and regions of different stiffness within the elastic host material. The concentration of light altering dopant may increase toward the center of the host material. The concentration of light altering dopant may also increase toward a preselected focal region inside the elastic host material. The optical element may include a gradient stiffness distribution within the elastic host material. The stiffness may decrease toward a preselected focal region inside the elastic host material. The concentration of the light altering dopant may increase toward regions of lesser stiffness. The light altering dopant may be a nonlinear absorbing chromophore. The nonlinear absorbing chromophore may be copper phthalocyanine. The light altering dopant may also be silicon (IV) 2,3-naphthalocyanine bis(trihexylsilyloxide). The elastic host material may be an epoxy resin.
The present invention is further directed to an optical element, comprising an elastic host material, a light altering dopant inside the elastic host material, and regions of different stiffness within the elastic host material. The optical element may include a gradient stiffness distribution within the elastic host material. The stiffness may decrease toward a preselected focal region inside the elastic host material.
The present invention is still also directed to a method for making an optical element, comprising the steps of providing an elastic host material and doping the elastic host material with a light altering dopant such that there is a nonuniform concentration distribution of the light altering dopant inside the elastic host material.
The present invention is still further directed to a method for making an optical element, comprising the steps of providing an elastic host material having a nonuniform stiffness distribution and doping the elastic host material with a light altering dopant such that there is a nonuniform concentration distribution of the light altering dopant inside the elastic host material.
The present invention is yet also directed to an optical limiter, comprising a first outer layer of a crosslinked polymer host material of a first stiffness, the first outer layer not including a light limiting dopant, a first inner layer of a low crosslink density crosslinked polymer host material next to the first outer layer, the first inner layer having a stiffness less than the first outer layer and including a light limiting dopant, a second inner layer of a low crosslink density crosslinked polymer host material next to the first inner layer, the second inner layer having a stiffness less than the first inner layer and including a light limiting dopant, and a second outer layer of a crosslinked polymer host material next to the second inner layer, the second outer layer having the same stiffness as the first outer layer, and the second outer layer not including a light limiting dopant.
The present invention is yet further directed to an optical limiter, comprising a plurality of layers of crosslinked polymer host material, wherein the stiffness of successive layers decreases from layer to layer from the outermost layers to the innermost layers and wherein a plurality of inner layers are doped with a light limiting dopant such that the amount of doping successively increases from layer to layer from the outermost of the inner layers to more innermost layers.
The present invention is moreover directed to a method for limiting the transmission of electromagnetic energy comprising placing in the path of the electromagnetic energy an optical limiter comprising a low crosslink density crosslinked polymer host material having an optical limiting dopant within the crosslinked polymer host material and a nonuniform concentration distribution of the optical limiting dopant within the crosslinked polymer host material.
The present invention is still moreover directed to a method for limiting the transmission of electromagnetic energy comprising placing in the path of the electromagnetic energy an optical limiter comprising a crosslinked polymer host material having an optical limiting dopant within the crosslinked polymer host material, a nonuniform concentration distribution of the optical limiting dopant within the crosslinked polymer host material, and regions of different stiffnesses within the crosslinked polymer host material. The concentration of light limiting dopant may increase toward the center of the crosslinked polymer host material. The stiffness of the crosslinked polymer host material may decrease toward a preselected focal region inside the crosslinked polymer host material. The concentration of light emitting dopant may also increase toward regions of lesser stiffness.